The present invention relates to the synthesis of novel liquid crystals, more particularly usable in liquid crystal display devices utilizing the electrically controlled birefringence effect. This effect corresponds to a deformation under an electric field of a nematic phase with negative dielectric anisotropy .DELTA..epsilon., .DELTA..epsilon. representing the difference between the dielectric constant .epsilon..sub.a parallel to the major molecular axis of the crystal and the dielectric constant .epsilon..sub.b perpendicular to said major axis.
In liquid crystal display devices, it is necessary to use materials having a high multiplexing rate, i.e. giving the possibility of electrically addressing a large number of rows of the screen, in order to display a high information rate. This multiplexing rate k can be expressed as a function of the voltage applied to the terminals of the device (V) and the threshold deformation voltage of the crystal (V.sub.S) by the formula: ##EQU1##
For a given multiplexing rate, it is necessary to have a molecular tilt angle (.phi..sub.M) at the centre of the liquid crystal cell which is as large as possible, in order to obtain an optimum contrast between the two black and white states of the crystal under the action of the electric field. For small tilt angles .phi..sub.M is directly linked with the threshold deformation voltage of the crystal V.sub.S and with respect to the flexion K.sub.33 and fanning K.sub.11 elastic constants of the liquid crystal. Thus, .phi..sub.M is given by the formula: ##EQU2##
Under these conditions, the electrically induced birefringence is of the form: ##EQU3## in which n.sub.e and n.sub.o are the extraordinary and ordinary indices of the crystal, e the liquid crystal thickness and Z its position in the cell.
The relative intensity transmitted by the liquid crystal device between two cross polarizers is given by the relation: ##EQU4## in which .DELTA.n is equal to n.sub.e -n.sub.o and .lambda. is the wavelength of the light beam illuminating the device. Therefore the most important parameters of the material are K.sub.33 /K.sub.11, .DELTA.n, .DELTA..epsilon. and the mesomorphism range .DELTA.T of the crystal.
The ratio K.sub.33 /K.sub.11 plays an important part among these parameters, because it determines the steepness of the slope of the electrooptical transfer curve and therefore the multiplexibility of the material used and said ratio must be as large as possible.
As a function of the nematic compounds used, this ratio can vary from 0.5 to 3, the most widely used value being approximately 1. In order to have a higher value of the ratio K.sub.33 /K.sub.11, consideration can be given to increasing the value of K.sub.33 or decreasing the value of K.sub.11.
A high value of K.sub.33 is of no interest in display devices, because it involves the use of high control voltages. Thus, the threshold deformation voltage of the crystal V.sub.s is proportional to the value of the elastic deformation constant in flexion K.sub.33 according to the formula: ##EQU5##
It is therefore preferably to reduce the value of K.sub.11, which corresponds to decreasing the intermolecular interactions between the molecules of the liquid crystal. Generally, the nematic liquid crystal molecules are formed by a rigid central part having phenyl groups and one or more flexible parts located at the ends of the rigid part.
Several solutions have been considered for reducing the interaction between molecules of this type. One of the solutions is to replace a phenyl cycle of the rigid part by a cyclohexane or a bicyclo-octane nucleus. Thus, as a result of the substantially globular shape of the latter nucleus and as a result of its completely saturated bonds, it does not favor intermolecular interactions and this makes it possible to minimize the value of K.sub.11 and increase the K.sub.33 /K.sub.11 ratio. A second solution is to substitute one or more hydrogen atoms of the phenyl nucleus by an electro-negative element, such as fluorine.
Recently, novel liquid crystals have been synthesized for which the K.sub.33 /K.sub.11 ratio has a high value. These novel liquid crystals are e.g.:
alkylfluorophenyl-alkylbicyclo(2,2,2)-octane carboxylates, like compound BCO55F complying with the following formula: ##STR4##
4-(trans-4-n alkylcyclohexyl)-4'-alkyl-2,2' difluorodiphenyls, such as compound BCH52FF of formula: ##STR5##
and bicyclohexylcarbonitriledialkyls or alkylalkylenes.
Such compounds have the following characteristics:
______________________________________ COMPOUND K.sub.33 /K.sub.11 n ______________________________________ 1. 0.13 -1 BCO55F 1.35 0.06 -0.75 BCH52FF 1.7 0.99 -0.47 ______________________________________
As can be seen from the above table the first line gives the characteristics required for display devices using electrically controlled birefringence, said compounds only being advantageous from the elastic constant standpoint, the value of the K.sub.33 /K.sub.11 ratio being high. However, their other characteristics (n and ) do not make it possible to obtain the requisite electrooptical properties for display devices. Therefore consideration has been given to the use of these compounds mixed with other liquid crystals for improving the electrooptical properties, but this leads to a significant drop in the value of the K.sub.33 /K.sub.11 ratio.